This invention relates to the use of wavefront sensing to determine whether or not a cornea has been altered (due to corrective surgery, or accident). More specifically, the present invention relates to the use of wavefront sensing using a distorted defraction grating to identify corneas (whether in vitro or in vivo) that have been surgically modified (e.g., radical keratotomy (RK), excimer laser photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK) and automated lamellar keratoplasty (ALK)).
In the United States about 40,000 corneal transplant operations are performed each year. While success of such surgery may depend upon a number of other factors, one factor that always has an effect on the outcome is the condition of the donor cornea. In the United States, a donor cornea must be transplanted within 7 days of harvesting. Outside the United States donor corneas may be used up to 14 days after harvesting. Additionally, it is essential to use only corneas which have not been modified (e.g., the subject of photorefractive surgery).
The growth of refractive surgery over the last five years has been dramatic. In the August 2000 issue of Archives of Ophthalmology, P. J. McDonnell, M. D. states that this year alone over 1,500,000 refractive procedures will be performed. As beneficial as these procedures are, the individual corneas are permanently altered, which makes them unsuitable for corneal transplanting.
The increase in refraction surgery increases the likelihood that a modified cornea will be harvested for transplant purposes. Unfortunately, it is generally not possible to conclusively tell, either visually or under a microscope, whether such a donor cornea has been subjected to a surgical procedure or otherwise altered.
Even when properly stored in a container (e.g., a Chiron Ophtholmics cornea container) filled with Optisol(copyright) or another appropriate solution, a donated cornea changes optically in the 14 day time period referenced above. The interior starts to develop optical scatter sources and the optical power of the cornea changes. The scatter resources manifest themselves as randomly distributed optical aberrations which increase over time. It is believed that this is caused by the cells of the harvested cornea not being able to reject waste material. The change in optical power is believed to be caused by an overall relaxation of the tissue. Regardless of the cause, the net result is that these aberrations produce scintillation and static aberrations when a beam of light is passed through a donated cornea.
PCT/GB99/00658 (International Publication No. WO 99/467768), based on applications filed in Great Britain on Mar. 10, 1998 and Dec. 23, 1998, discloses a three dimensional imaging system including a lens and a distorted diffraction grating which images objects located at different distances from the grating simultaneously and spatially separated in a single image plane. The grating is distorted according to a quadratic function so as to cause the images to be formed under different focus conditions. It is stated that the system is useful for simultaneously imaging multiple layers within a three dimensional object field, and has applicability in a number of fields including optical information storage, imaging short-time scale phenomena, microscopy, imaging three dimensional object structures, passive ranging, laser beam profiling, wavefront analysis, and millimeter wave optics. The ability to make wavefront measurements is not disclosed or claimed.
P. M. Blanchard et al., xe2x80x9cMulti-Plane Imaging With a Distorted Grating,xe2x80x9d Proceedings of the 2nd International Workshop on Adaptive Optics for Industry and Medicine, World Scientific, pp. 296-301, Jul. 12-16, 1999, describe a technique for simultaneously imaging multiple layers within an object field onto the detector plane of a single detector. The authors, who are the named inventors in PCT/GB99/00658, state that the imaging of multiple layers within an object field is xe2x80x9cuseful in many applications including microscopy, medical imaging and data storage.xe2x80x9d (See page 296.) The apparatus includes the use of a binary diffraction grating in which the lines are distorted such at each different level of defocus is associated with each diffraction order. When such a grating is placed in close proximity to a lens, the grating creates multiple foci of the image. This multi-foci effect enables the imaging of multiple object planes onto a single image plane.
L. J. Otten et al. xe2x80x9c3-D Cataract imaging System,xe2x80x9d Proceedings of the 2nd International Workshop in Adaptive Optics for Industry and Medicine, World Scientific, pp. 51-56, describe optics and an associated diagnostic system for volumetric, in vivo imaging of the human lens to characterize or grade cataracts. The described method and apparatus are based on the use of a distorted grating (of the type disclosed in PCT/GB99/00658 and the Blanchard et al. paper, supra) in conjunction with a focusing lens and a re-imaging lens. (See FIG. 1 of this reference.) The quadratic phase shift, introduced by the grating, leads to a different degree of defocus in all diffraction orders, which produces a series of images of different layers of the cataract, each with different defocus conditions, simultaneously and side-by-side on the detector. Thus, in-focus images of different object planes are produced.
Analysis of the optical images referenced above requires the use of the Intensity Transport Equation (I.T.E.) and the employment of a Green""s function to produce a wavefront map. S. Woods, P. M. Blanchard and A. H. Greenaway, xe2x80x9cLaser Wavefront Sensing Using the Intensity Transport Equation,xe2x80x9d Proceedings of the 2nd International Workshop on Adaptive Optics for Industry and Medicine, World Scientific, pp. 260-265, Jul. 12-16, 1999, describe both the I.T.E. and a Green""s function solution thereto in conjunction with laser wavefront sensing.
It is an object of the present invention to determine, with wavefront sensing, whether or not a cornea has been altered (either deliberately or accidentally).
It is another object of the present invention to determine, with the use of wavefront sensing using a distorted grating, those donor corneas that have been modified by surgery or other methods.
It is another object of the present invention to provide a simple optical system (particularly including a light source, an imaging lens, a distorted grating and a data camera) to form, in the detector plane, images from which wavefront aberrations in the cornea can be derived. The beam of light that passes through a cornea (located in the pupil plane) and two virtual planes on opposite sides of and equidistant from such pupil plane.
It is an additional object of the present invention to provide a holder for a donor cornea which does not mask optical data from such cornea.
It is yet another object of the present invention to provide a holder for a donor cornea that has optical windows that are substantially free of distortion which would mask corneal optical data.
It is yet still another object of the present invention in which the optical windows have less than xcex/10 distortions.
These and other objects will be apparent from the description which follows.
A method of determining if a cornea (whether in vitro or in vivo) has been modified (either surgically or otherwise). The method includes the steps of: passing a beam of collimated light a (either coherent or incoherent) through the cornea to produce a distorted wavefront; determining the characteristics of the distorted wavefront; and analyzing the distorted wavefront for characteristics that identify the presence of a modification. The analysis of the distorted wavefront can be for the presence of higher order aberrations, or Gausian characteristics which are indicative of modifications. More particularly, the method includes the steps of providing an optical system that has a pupil plane and an image plane at a detector; positioning the cornea in the pupil plane; passing a collimated beam of light through the cornea to produce at least two images in the image plane; determining the characteristics of the distorted wavefront; and analyzing the distorted wavefront for characteristics that identify the presence of a modification.
The apparatus for determining whether a cornea has been surgically modified includes: a source of collimated light, an optical system including a distorted grating and an imaging lens (which have a pupil plane, first and second virtual planes, and an image plane); structure for positioning the cornea in the pupil plane; means for recording the images of the first and second virtual planes; means for determining from the first and second images the distorted wavefront; and means for analyzing said wavefront for characteristics indicative of modified corneas. The first and second virtual planes are on opposite sides of and equally spaced from said pupil plane.
The structure for positioning the cornea (which is immersed in a suitable storage fluid) is a container which includes: a housing having first and second ends; structure positioned within the housing for supporting the perimeter of the cornea; a first plano/plano lens for closing the first end of the housing; and a cap for closing the second end of the housing. The cornea support is substantially symmetrical with respect to an optical axis. The first plano/plano lens is substantially perpendicular to the optical axis. Finally, the cap includes a second plano/plano lens which is substantially parallel to the first plano lens. The first and second plano/plano lens, which are substantially centered with respect to the axis, have less than xcex/10 total distortions. The support structure includes a cage and a pedestal with the cage being supported by the pedestal. Preferably, the cage and pedestal are integrally formed with the housing. Finally, the cap has a top portion and a skirt which axially inwardly spaces the second lens from the top portion to create an annular area where air will collect when the container is holding a cornea and fluid, so that air will not interfere with a beam of light passing through the first and second plano/plano lenses and cornea.